What is generally perceived as a very important parameter in circuit design is a device's transconductance, “gm”. Transconductance (short for “transfer conductance”, and also infrequently called “mutual conductance”) is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciprocal of resistance. Calculating gm is important when conducting AC (alternating current) analysis of a transistor circuit. “gm*Vbe” represents the gain of the AC signal at a transistor after it undergoes amplification. Gain can also be represented as “gm*Vgs” in the case of metal oxide semiconductor (MOS) transistors.
Quadrature signals, also called IQ signals, IQ data or IQ samples, are often used in radio frequency (RF) applications. They form the basis of complex RF signal modulation and demodulation, both in hardware and in software, as well as in complex signal analysis. Polyphase/image rejection (IR) filters are widely used in wireless receivers when I and Q signals are required. They generally operate with polyphase/image reject mixers.
Various implementation methods are available for on-chip filters, including active RC (resistor-capacitor), active RLC (resistor-inductor-capacitor), MOSFET-C (metal-oxide-semiconductor field-effect transistor-capacitance), gm-C and switched capacitor. Using a low noise transconductance, gm-C implementation is believed to have the best power consumption versus noise trade-off and presents good high frequency performance. Using active RC filtering has been popular for high linearity applications (e.g., WI-FI 802.11). RC filters also provide much better Image Rejection (IR), in the range of 15-50 dB depending on the application and design requirements. Coupling components are typically provided in the form of resistors for typical RC filters.
Gm-C filters are good for low linearity, low noise and high-speed applications. They take less area and consume less power than other filtering components. Component mismatch is common, however, because coupling components that turn low pass filters to polyphase (Band Pass) filters are of the gm variety (therefore “gmcomponents”), which are active components instead of active RC (resistor-capacitor) polyphase filters which use passive components. This can be a problem in that coupling gm components tend to achieve less IR compare to active RC polyphase filters in range of 7-25 dB (i.e., lower IR). This problem is common in Gm-C filtering circuitry because different types of, or values for, gm cells are used.